[mlir][linalg] Fix bug in the fusion on tensors index op handling.

The old index op handling let the new index operations point back to the
producer block. As a result, after fusion some index operations in the
fused block had back references to the old producer block resulting in
illegal IR. The patch now relies on a block and value mapping to avoid
such back references.

Differential Revision: https://reviews.llvm.org/D101887